Environmental regulations covering the sulfur content of fuels for internal combustion engines are becoming more stringent with regard to allowable sulfur in fuels. Motor gasoline sulfur content will need to meet a sulfur limit of 30 wppm between 2004-2006 with possible further reductions mandated in the future. One of the main components of motor gasoline is typically catalytically cracked naphtha, which contains substantial amounts of sulfur and olefins.
A common method for reducing the sulfur content of catalytically cracked naphtha feedstocks is by hydrotreating using catalysts that convert sulfur-containing species to hydrogen sulfide. The extent to which hydrotreating lowers the sulfur content of the hydrotreated product is typically dependent on the catalyst and hydrotreating conditions. For any given hydrotreating catalyst, the more severe hydrotreating conditions would be expected to reduce the sulfur content to the greater extent. However, such severe hydrotreating conditions normally result in a loss of molecules contributing to desirable octane properties either by cracking to non-fuel molecules or hydrogenation of olefins to molecules having lower octane rating. As the hydrotreating catalyst ages, it normally becomes necessary to adjust reaction conditions to maintain an acceptable catalyst activity. However, such adjustments result in further loss of desirable molecules contributing to high octane. This then results in increased production costs to produce high octane fuels because of the need to boost octane through added process steps such as isomerization, blending or addition of octane boosting additives.
One approach to addressing the problems associated with conventional hydrotreating is to use selective hydrodesulfurization, i.e., hydrodesulfurizing a feed with selective catalysts, selective process conditions, or both, to remove organosulfur while minimizing hydrogenation of olefins and octane reduction. For example, Exxon Mobil Corporation's SCANfining process selectively desulfurizes cat naphthas with little or no loss in octane number. U.S. Pat. Nos. 5,985,136; 6,013,598; and 6,126,814, all of which are incorporated by reference herein, disclose various aspects of SCANfining. Although selective hydrodesulfurization processes have been developed to avoid significant olefin saturation and loss of octane, H2S liberated in the process can react with retained olefins to form mercaptan sulfur by reversion. Such mercaptans are often referred to as “recombinant” or “reversion” mercaptans.
It is known that hydrotreating can be followed by additional steps such as adsorption or liquid extraction for mercaptan removal. An example of such post-hydrotreatment mercaptan removal is U.S. Pat. No. 6,228,254.
There is still a need to improve the sulfur removal process from feeds.